Ethernet transceivers include a Media Access Control device (MAC) and a Physical Layer Device (PHY) coupled by a Media Independent Interface (MII). The MAC layer is responsible for, among other things, controlling access to the media, and the PHY layer is responsible for transmitting bits of information across a link. In Ethernet the interface between the MAC and the PHY is specified by IEEE 802.3 and has evolved from the 10 pin MII (Media Independent Interface) for 10/100 Ethernet, to the 20 pin GMII (Gigabit Media Independent Interface) for GE, to the 36 pin XGMII (10 Gigabit Media Independent Interface) for 10GE along with other variants such as SMII, SGMII, XAUI and others.
IEEE P802.3az is currently defining a new Energy Efficient Ethernet mode of operation for multiple Ethernet PHYs. In particular 100BASE-TX, 1000-BASE-T, 10GBASE-T and some backplane PHYs will be modified to support a Low Power Idle (LPI) mode that allows the PHY and other system components to save energy during periods of low network traffic load.
This energy saving is achieved when using the LPI state because the 802.3az-compliant PHY's activity level may drop after it is instructed to enter the LPI state and it will receive ample warning to exit the LPI from 802.3az-compliant host side hardware (e.g. 802.3az-compliant MAC and host ASICs) before live network traffic is expected.